Jurors in the federal death penalty trial of Boston Marathon bomber Dzhokhar Tsarnaev began deliberations Tuesday, a day after both prosecutors and his lawyers told them Tsarnaev must be held accountable for participating in the terror attack.

Deliberations in the guilt phase began almost two years after twin bombs exploded near the marathon’s finish line on April 15, 2013, killing three people and wounding more than 260.

During closing arguments Monday, Tsarnaev’s lawyers agreed with prosecutors that Tsarnaev conspired with his brother to bomb the marathon and planted one of two pressure-cooker bombs that exploded that day.

But the defense said it was his now-dead older brother, Tamerlan, who was the mastermind of the attack. It was Tamerlan who bought the bomb parts, built the bombs and planned the attack, said defense attorney Judy Clarke.

A prosecutor told the jury that Tsarnaev made a coldblooded decision aimed at punishing America for its wars in Muslim countries.

“This was a cold, calculated terrorist act. This was intentional. It was bloodthirsty. It was to make a point,” Aloke Chakravarty said. “It was to tell America that ‘We will not be terrorized by you anymore. We will terrorize you.’”

Clarke argued that Tsarnaev fell under the influence of Tamerlan. Clarke repeatedly referred to Dzhokhar Tsarnaev — then 19 — as a “kid” and a “teenager.”

Prosecutors used their closing to remind the jury of the horror of that day, showing photographs and video of the carnage and chaos after the shrapnel-packed pressure-cooker bombs exploded. In one video, jurors could hear the agonizing screams of Krystle Campbell, a 29-year-old restaurant manager who bled to death on the sidewalk. Another woman and an 8-year-old boy also were killed.

Taking aim at the argument that Tsarnaev was led astray by his older brother, Chakravarty repeatedly referred to the Tsarnaevs as “a team” and “partners” in the attack.

“That day, they felt they were soldiers. They were the mujahedeen, and they were bringing their battle to Boston,” the prosecutor said.

Tamerlan Tsarnaev, 26, died four days after the bombings after he was shot by police and run over by Dzhokhar during a getaway attempt. Dzhokhar was captured hours later hiding in a dry-docked boat.

If Tsarnaev is convicted — and that is considered a near certainty, given his lawyer’s admission — the jury will then begin hearing evidence on whether he should get life in prison or a death sentence.